Syncopal events and arrhythmias of the heart are particularly problematic for diagnostic physicians to observe in living patients. These events, can be of short duration and sudden onset, coming with little or no warning, and may happen very infrequently. Holter monitors are well known for monitoring electrocardiograms periods of time amounting to days or perhaps a week, but these are bulky and are applied externally to the body and interfere with the patient's normal life, making them impractical for long term use. Further, patient compliance cannot always be guaranteed, and is a common problem in use of the Holter devices. Problems with external monitors and associated recorders also include inability of some patients to abide the attendant skin irritation. Bulky or expensive special purpose devices may need to be available and maintained. Removal is required for showering, and so on. Any time a living body needs to have a long term monitoring of a physiologic event that is intermittent or infrequent or both, all these problems come into focus. Therefore, there exists a need for minimally intrusive long-term monitoring of the patient's physiologic events and status. This is particularly indicated in, but not limited to patients with cardiac arrhythmias and vasovagal syncope to provide sufficient evidence for diagnostic purposes and for research into the causes and effects of such events. Patients have come to accept long term implants of small items for many things, including birth control, for example, like the "Norplant" (.TM. of Wyeth Laboratories) devices which secrete birth control hormones for perhaps a year before they need replacing. Accordingly it is believed that small device implants for long term implant will be well tolerated by the patient population to be served by this invention.
Many attempts to address some of these problems have been made and met with limited success. The problem has been long existing. The Instromedics approach is seen in the Mills, et al patents (U.S. Pat. Nos. 5,333,616; 5,289,824 and 5,111,396) for a wrist worn monitor for ECG's which include features like patient triggering and microprocessor determination of event types (QRS detection). Wrist worn devices are also shown in the Righter patents issued to assignee Ralin, including U.S. Pat. Nos. 5,226,425 and 5,365,935. Jacobsen, et al in U.S. Pat. No. 5,513,645 describes multiple resolution storage for ECG's (ELA Medical is the assignee), and Snell's U.S. Pat. No. 5,518,001 vaguely describes a patient triggered recording device with multiple sensors and patient triggering(assigned to Pacesetter). InControl's approach is seen in the Yomatov patents, U.S. Pat. Nos. 5,411,031 and 5,313,953 which seems to concentrate on beat to beat timng records, suggests the use of an arrhythmia detector, and does mention the possibility of leadless electrodes for monitoring cardiac signals. Examples of an external monitor/recorders can be found in Segalowitz' patents, including U.S. Pat. No. 5,511,553, and Salo's U.S. Pat. No. 5,417,717. Another well known event recorder is the "King of Hearts" (.TM. of Instramedix) which records pre-event and post-event data.
Monitoring can be done using implantable pulse generators such as pacemakers and other heart stimulating devices or devices with leads in the heart for capturing physiologic parameters, including the ECG. However, the expense and risk from implanting an intracardiac lead and/or a pacemaker with special monitoring functions is something both patients and physicians would prefer to avoid. Such devices, in addition to performing therapeutic operations, may monitor and transmit cardiac electrical signals (e.g., intracardiac electrograms) to an external diagnostic devices typically with leads fixed in the patient's heart, to observe electrical activity of a heart. It is common for implanted cardiac stimulation devices to send intracardiac electrogram signals to a monitoring device, such as an external programmer, to allow a user to analyze the interaction between the heart and the implanted device. Often the user can designate that the communication from the implantable device to the programmer include a transmission of codes which signal the occurrence of a cardiac event such as the delivery of a stimulation pulse or a spontaneous cardiac depolarization.
For example, U.S. Pat. No. 4,223,678, entitled "Arrhythmia Recorder for Use with an Implantable Defibrillator", issued to Langer et al. on Sep. 23, 1980, discloses an arrhythmia record/playback component within an implantable defibrillator. ECG data is converted from analog to digital (A/D) form and stored in a first-in, first-out memory. When the defibrillator detects an arrhythmia event, it disables the memory so that no further ECG data is recorded in the memory until a command is received from an external monitoring device. This command requests the implantable defibrillator to transmit the stored ECG data to the monitoring device via telemetry. Langer et al. in U.S. Pat. No. 4,407,288, entitled "Implantable Heart Stimulator and Stimulation Method", issued Oct. 4, 1983, discloses a programmable, microprocessor based implantable defibrillator which senses and loads ECG data into a memory via a direct memory access operation. A processor analyzes this ECG data in the memory to detect the occurrence of an arrhythmia event afflicting a patient's heart. Upon such an event, the defibrillator may generate a therapy to terminate the arrhythmia event and store the ECG data sequence of the event, for transmission to an external monitoring device and later study. In normal circumstances, when no arrhythmia event is occurring, the defibrillator continuously overwrites the ECG data in the memory.
U.S. Pat. No. 4,556,063, entitled "Telemetry System for a Medical Device", granted to D. L. Thompson et al, 1985, teaches a pulse interval telemetry system capable of transmitting analog data, such as sensed intracardiac electrogram signals, without converting analog data to a digital numeric value. The Thompson et al. telemetry system is capable of sequentially transmitting both digital and analog data, individually and serially, in either an analog or a digital format, to a remote receiver. The features and capabilities of these pacemaker/defibrillator devices is now well known, but the problems in long term monitoring for events and adequate recordation remain.
In the December 1992 Vol. 15 edition of PACE (15:588), a feasibility study was done for implantable arrhythmia monitors and reported in an article by Leitch et al. Subcutaneous, Bipolar "Pseudo-ECG" Recordings using an Implantable Monitoring System and at chaired poster presentation of the North American Society of Pacing and Electrophysiology (NASPE) an implantable monitoring system was described using the pacemaker that had been altered to use a point on the can as an electrode and to have an electrode mounted into the connector block thereof. This was presented to NASPE in Munich in 1994 by Brian Lee of Medtronic, Inc. A photograph of the device shown in that poster presentation was published by the American Heart Association Inc. in 1995 by Andrew Krahn, M.D. in an article entitled "The Etiology of Syncope in Patients with Negative Tilt Table and Electrophysiological Testing", pp. 1820 of CIRCULATION, 1995; 1992. The initial thinking for this started in NASPE 1991 in an Abstract published in PACE, 1991, 14:677 authored and titled: Leitch, J W, Klein, G J, Yee, Lee B B, Kallok, M, Combs, B, Bennett, T: Feasibility of an Implantable arrhythmia Monitor.
Further, a leadless implantable sensor for cardiac emergency warning was described in U.S. Pat. No. 5,404,887 issued to Knowlan et al. which detects heart events through impedance measurement sensed using a coil. See also Yomato et al, U.S. Pat. No. 5,313,953 which describes (in FIG. 26) a large but leadless implant.
With sufficient hardware and connections to the body, numerous other physiologic parameters may be sensed as is pointed out in U.S. Pat. No. 5,464,434 issued to Alt and U.S. Pat. No. 5,464,431 issued to Adams et al.
Accordingly, there still exists a need for a more acceptable recording and monitoring device capable to maintain a data record over a long period of time and highlighting or least capturing those physiologic events that are of interest to a diagnostic, research or therapeutic study, and particularly those physiologic events that are required for correct diagnosis and therapy. Further, it has heretofore been unreasonably expensive and overly invasive to the patient to implant monitors for simple recording functions and particularly to implant intracardiac and intravascular monitors for simple recording functions. Many of the features of this invention are designed to ameliorate both these problems.